A weak cosmic infrared radiation field that reaches Earth from all
directions contains not yet deciphered messages about the evolution of
galaxies. Using first observations with the PACS Instrument on board
ESA's Herschel Space Telescope, scientists from the Max Planck Institute
for Extraterrestrial Physics and other institutions have for the first
time resolved more than half of this radiation into its constituting
sources. Observations with Herschel open the road towards understanding
the properties of these galaxies, and trace the dusty side of galaxy
evolution.

Herschel-PACS images of the 'GOODS-N' field in the constellation of
Ursa Major at far-infrared wavelengths of 100 and 160 µm. Galaxies at
high redshift (i.e. larger cosmological distance) or with colder dust
are displayed in red, while nearby galaxies appear in blue.
Image: MPE

In the mid 1990's, scientists analyzing data from NASA's COBE spacecraft
discovered faint radiation in the far-infrared part of the electromagnetic
spectrum that reaches earth with the same intensity from all directions in
space. Immediately, they suspected it to be the aggregate emission of many
distant galaxies in the early universe, releasing the same amount of energy
in the far-infrared as reaches us in visible light from similarly distant
galaxies. Whereas visible light tells us about the stars in galaxies, the
far-infrared is emitted by cold dust that is hiding the newly formed stars.
Identifying these surprisingly numerous dusty galaxies has proven difficult,
though. Space telescopes are needed to detect far-infrared emission, because
it is absorbed by the Earth's atmosphere. Previous infrared space telescopes
have detected far-infrared light from only the brightest of the galaxies
forming this cosmic background. To glean any information about the fainter
objects, astronomers had to rely on indirect evidence based on shorter
wavelength radiation.

ESA's Herschel Space Observatory, launched in May 2009, is the largest space
telescope ever built with a mirror diameter of 3.5m. Its PACS instrument is
designed to take high-resolution images of the sky at far-infrared wavelengths
of 70 to 160 µm, exactly where most of the cosmic infrared background emission
is received. "After the check-out of our instrument, we were yearning to obtain
the first deep far-infrared observations of the sky," says Albrecht Poglitsch,
principal investigator of PACS.

For a total of 30 hours in October, PACS has observed a small patch of sky in
the constellation of Ursa Major, about a quarter of the size of the full moon.
"Already in these first observations, we have resolved about 60% of the cosmic
infrared background from this region of sky into individual well-detected sources,"
says Dieter Lutz from the consortium of scientists from five European institutes
that have obtained the data. "And this is just the beginning. Yet more sensitive
observations will follow soon, and we will be able to understand in detail the
epoch of activity and the properties of the galaxies that produce the cosmic
infrared background, now that we have pinned them down."

The PACS images of the GOODS-N field were obtained as part of the Herschel
Guaranteed Time Key Programme "PACS Evolutionary Probe" (PEP) by a consortium
which includes scientists from Max Planck Institute for Extraterrestrial
Physics (Germany), CEA Saclay (France), Instituto de Astrofísica de Canarias
(Spain), Istituto Nazionale di Astrofisica (Italy), and the Herschel Science
Centre, led by Dieter Lutz (MPE Garching).

The PACS instrument has been designed and built by a consortium of institutes
and university departments from across Europe under the leadership of Principal
Investigator Albrecht Poglitsch located at the Max Planck Institute for
Extraterrestrial Physics in Garching, Germany. Consortium members are:
Austria: UVIE; Belgium: IMEC, KUL, CSL; France: CEA, OAMP; Germany: MPE,
MPIA; Italy: IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA; Spain: IAC; Hungary:
Konkoly; USA: NHSC.